Industrial linear actuators are commonly used to perform a variety of functions, such as linearly translating a locating pin or operating a clamp for maintaining a position of a workpiece. A typical linear actuator comprises a housing having linearly-translating shaft operably coupled to a drive means, such as a pneumatic piston and cylinder arrangement or a geared electric motor. In many applications, precise positioning of the linearly-translating shaft is essential to maintaining specific tolerances in a final assembly of the workpiece.
It is typically desirable that the shaft not rotate with respect to the housing, but rather, extend in a straight line along a single axis. Thus, it is desirable that the yaw, pitch, and roll of the shaft with respect to the linear translation be minimized. Accordingly, many attempts have been made to accurately position the shaft with respect to the housing, wherein various mechanisms and shaft designs have been used to prevent such yaw, pitch, and roll. One example is illustrated in FIG. 1, wherein a conventional linear actuator 10 is provided having a square shaft 15 that extends and retracts with respect to a housing 20 for positioning a workpiece (not shown). The housing 20, is provided with a square bore 25, wherein a sacrificial square bearing 30 guides the shaft 15 throughout its extension and retraction. The sacrificial square bearing 30 is typically comprised of a material that is substantially softer than the square shaft 15, thus allowing the square bearing to wear more quickly than the typically more-expensive square shaft. Typically, the sacrificial square bearing 30 needs to be replaced on a regular basis, whereas the square shaft 15 may last significantly longer without requiring replacement.
One problem with the sacrificial square bearing 30 tending to wear with time, however, is that the wear on the square bearing typically leads to a potential pitch, yaw, and roll of the square shaft 15 with respect to the housing 20 due to increased slop between the shaft and the square bearing. Thus, inaccuracies in positioning of the shaft 15 with respect to the housing 20 tend to increase as the usage of the linear actuator 10 increases, thus leading to potential production losses due to missed tolerances on the workpiece.
Furthermore, in harsh environments, such as a weld shop or metal cutting environment, contaminants 35, such as weld spatter or metal chips, can affix themselves to the shaft 15 when the shaft is extended from the housing 20. When the shaft 15 is retracted back into the housing 20, the contaminants 35 that are affixed to the shaft can cause further wear and damage to the square bearing 35, thus decreasing the lifespan of the square bearing even further. In order to alleviate some of the additional wear induced by harsh environments, it has been conventional to cover an exposed portion 40 of the shaft 15 with a shroud or boot 45, wherein the shroud or boot generally prevents the contaminants 35 from contacting the exposed portion of the shaft. Such shrouds or boots 45, however, tend to make the linear actuator 10 bulky and cumbersome, and further tend to increase a total length of the linear actuator due to the additional space needed to affix the shroud or boot to the housing 20.
Accordingly, a need exists in the art for a reliable, low-maintenance linear actuator that provides accurate positioning of the shaft over a substantially longer period of use than previously achieved. Such a linear actuator should overcome, or at least minimize, the above-described drawbacks. Preferably, the linear actuator would comprise a simple and economical, yet reliable, device that would accurately position the shaft with a minimum of wear to the linear actuator over its lifetime.